The present invention relates to a fuse element structure made of conductive metal and used in a semiconductor device.
With semiconductor elements integrated onto a semiconductor substrate (chip) in semiconductor devices becoming small, so that a huge number of such elements are included on a single semiconductor chip, the defect density has also increased, and low yield is a problem at the device development or initial mass production stage. To solve this problem, redundancy circuit techniques have been proposed and have come into practical use. A redundancy circuit that repairs defects that occur in the processes of manufacturing memory elements in particular will now be described. When a defect exists in a row, a column, or a cell that is part of an arrangement of memory cells, it is possible, by making available a number of spare rows or columns, to select one or more spare rows or spare columns when an address signal is input that corresponds to the defective part, thereby achieving operation as a normal device, even though the device has a defect. By incorporating this kind of redundancy circuitry, although the surface area of the chip increases, the yield is improved. In implementing this kind of redundancy circuit, it is important to select a means of what can be called programming, which assigns spare parts to addresses corresponding to defect locations which occur in a random manner from chip to chip. While a variety of such means exists, the most commonly used is that of laser-fusible elements, because of the small increase in chip surface area and process margin that is offered by this method. Fuse elements are often used in such applications in semiconductor devices.
While semiconductor devices having fuse elements according to the prior art are often made of polycrystalline silicon, because with polycrystalline silicon the contact resistance at a contact portion with the polycrystalline silicon substrate is high, recent years have seen an increase in the use of aluminum (Al) and tungsten (W), which are metals having low resistance. The shape is often a single bar or a shape having the part that is to be opened, or blown. The generally used means for blowing the fuses is the application of a laser beam.
In a semiconductor device having fuse elements according to the prior art, problems have arisen with the change of fuse element material from polysilicon to conductive metals such as Al, W, TiN/Ti/W (three layer structure of titanium nitride, titanium and tungsten) films. This problem is particularly prominent with a multilayer structure using TiN/Ti/W films.
FIGS. 1A and 1B will now be used to describe the fuse element part of a semiconductor device in the prior art. FIG. 1A is a top view of the fuse element part of a row fuse selector which is connected to a spare row decoder of a semiconductor memory such as a DRAM (Dynamic Random Access Memory), while FIG. 1B is a cross-sectional view of the part indicated by the line 1B--1B in FIG. 1A. In FIGS. 1A and 1B, three fuse elements 1 are shown, both ends of each fuse element being connected to semiconductor devices which form a desired circuit. Note, however, that such devices are hidden by an insulating film 4 and a passivation film 6, and so are not shown in FIG. 1A of the drawing. What is shown is only a hole (a fuse window 2) which is formed in part of the insulating film 4 and the passivation film 6. The shape of the fuse element, as shown in FIG. 1A, is that of a single bar, this being the generally used shape. Referring to the cross-sectional view presented in FIG. 1B, the fuse elements 1 are disposed over a semiconductor substrate 3 via the insulating film 4 of SiO.sub.2, and over which element is also formed SiO.sub.2 or the like as the insulating film 4.
Each fuse element 1 is made from a TiN/Ti/W film, this being a TiN/Ti (two layer structure of TiN and Ti) film 102, on top of which is formed a tungsten (W) film 101.
To blow this fuse element, a laser beam is generally used, laser beam being aimed at the area P surrounded by the circle in FIG. 1A in the direction of the arrow X as shown in FIG. 1B to blow the fuse element.
The condition of a fuse element of the prior art after it is blown by the above-noted method will now be explained with reference to FIGS. 2A to 2D and 3.
These figures show in cross section the area corresponding to the area P as shown in FIG. 1A. First, FIG. 2A is a cross-sectional view of the initial condition, with FIG. 2B showing in cross section the condition when the laser beam first strikes. Immediately after the laser beam strikes in the direction of the arrow X, the SiO.sub.2 insulating film 4 and the tungsten film 101 are destroyed and blown away. However, because the TiN/Ti film 102 is at the bottom surface, it remains, and is melted by the energy of the laser beam. In addition, the energy of the laser beam causes the melted TiN/Ti film 102 to begin to move in the direction of the arrows Z shown in FIG. 2C, so that it moves from the bottom surface of a groove 200 to accumulate at the corners of the bottom surface. Then, as this movement to the side surfaces proceeds, the TiN/Ti film 102 remains from the corners of the bottom of the groove 200 extending to the side surfaces, as shown in FIG. 2D. As shown in FIG. 3, a top view corresponding to FIG. 2D, the fuse element is not completely blown. Hence this is the problem existing in the prior art.